Infant mammals devote considerable resources to thermal, fluid, and cardiovascular homeostasis. Recent work in the PI's laboratory has shown that infant rats, long considered poor thermoregulators because of their high rates of heat loss during cold exposure, actually exhibit many signs of successful thermoregulation when tested under appropriate conditions. Their success depends on the ability to produce heat internally using brown adipose tissue (BAT), as well as the delivery of this heat to the heart and other temperature-sensitive organs. We have learned that infants that possess endothermic capabilities (i.e., rats) exhibit significantly different behavioral and physiological responses to cold than infants that do not (i.e., hamsters). We still do not understand, however, how infants of either species orient and locomote toward warmth and whether the ability to produce heat internally interferes with the expression of thermoregulatory behavior. Therefore, this proposal represents the logical next step in a research program that addresses basic issues in biobehavioral research and aims to develop a better understanding of the myriad physiological and behavioral mechanisms by which infants regulate their internal thermal environment and select their external thermal environment. This work has important implications for the thermal management of preterm and full-term human infants, sick or healthy, who differ in their abilities to produce heat endogenously and about whose thermoregulatory behavior we know very little. First, with our new appreciation of the thermoregulatory capabilities of individual infant rats, the behavioral and physiological responses of huddling rat pups during cold exposure are examined. Group regulatory behavior is examined in infant rats after ganglionic blockade, after selective activation of BAT thermogenesis in ganglionically blocked pups, and in mixed huddles comprised of infant rats and hamsters. These experiments will reveal how endothermy contributes to the expression and effectiveness of huddling behavior. Second, thermoregulatory behaviors in isolated individuals are examined using a newly- developed, novel apparatus - a multi-tiled "checkerboard" apparatus composed of peltier diodes - that provides fine control over the thermal environment. In addition, the use of infrared thermography provides essential thermal data without interfering with behavioral expression. The combination of these approaches places us in an ideal position to critically examine behavioral arousal, locomotion, orientation, and thermal preference in infant rats and hamsters during cold exposure, as well as the contributions of endothermy to each of these behavioral processes. Finally, subsequent experiments will address the sensory and neural mechanisms that mediate behavioral responses to cold.